The Cassini spacecraft performed a close flyby of Saturn’s moon Enceladus on Wednesday, passing within 50km of the surface at closest approach. Scientists are particularly interested to learn more about the icy plumes, rising from the south pole of Enceladus, that were discovered by Cassini in 2005. The spacecraft passed 200km above the south polar region this week – hopefully close enough to pass through the plumes and tell us more about these strange icy geysers. You can learn more about Enceladus in the following video from the Cassini web site, and read about the flyby in detail on the flyby blog. There is also a section of the Cassini web site where you can browse the raw images from the flyby.

The Moon has reached first quarter in its moonthly orbit around the Earth (a small mis-spelling of the word month, and you realise where the word originates from!). You have about 6 hours to admire the moon after sunset, until it too sinks below the western horizon, due to the relentless rotation of the Earth.
Most of us are used to the familiar sight of the first-quarter Moon through our own eyes. But the sight through an X-ray telescope is perplexing, to say the least.
As described in my previous post, X-rays originate from very hot gas. In the case of the Moon, the X-rays originate from hot gas bursting out of the Sun, which are then reflected off the Moon.
In 1990, the Rosat observatory took this image of the Moon.

It is somewhat ironic that X-ray astronomy began in the early 1960s, with the aim of detecting X-rays from the Moon. The plan was to get some clues about the conditions on the lunar surface before Neil and Buzz got there before the decade was out.

But it took another 30 years to detect those X-rays from the Moon!
Even more ironically, it turned out that the Moon wasn’t the really interesting bit after all. Look at the X-ray dark half of the Moon, and you notice that it is actually darker than the deep space behind it…
And that raises the question – where are all those X-rays behind the Moon coming from? (The dark side of the Moon is not completely black, as some charged particles from the Sun do collide with the far side of the Moon, producing X-rays.)
To know what was causing the X-ray background, we had to wait for Chandra, which showed that at least 80% of the X-rays originated from distant star-swallowing black holes – half of which are about the mass of the Sun, the other half being a billion times larger! (see here)
The image taken by Chandra is both boring and astounding at the same time! Boring, since the image is just a collection of dots. Astounding when you realise just what those dots actually are.
So how many black holes does the full Moon cover?
Well, it depends how sensitive your X-ray telescope is and for how long you look! But there will be anything between 100 and 1000, depending on where you look in the sky (it is easier to see black holes looking directly out of our galaxy).

Promethei Planum, near the south pole of Mars

Credit: ESA/ DLR/ FU Berlin (G. Neukum)

The European Space Agency (ESA) released a beautiful photo from the Mars Express spacecraft today. The photo above shows Promethei Planum, a region near the south pole of Mars which is covered by up to 3500m of ice during the Martian winter.

On the left of the photo, we can see a broad sheet of ice, which is an extension of the south polar ice cap. The steep flanks clearly show white, clean ice. The thickness of the ice is between 900 and 1100m. To the right of the ice sheet are structures that may have been created by basaltic lava flow from a volcano. The dark dunes towards the bottom of the image are most likely made up of dust originating from this lava flow or volcanic ash. Finally, the far right (or northern) side of the photo contains an impact crater, which is approximately 100 km wide and 800 m deep. The crater’s interior is partly covered in ice.

More photos are available from the ESA Mars Express web site.

Artist’s impression of rings around Rhea
Credit: NASA/JPL

The Cassini spacecraft has detected what may be evidence for rings around Rhea, Saturn‘s second largest moon. This is the first time that rings have been found around a planet’s moon. The discovery is the result of work by a team led by Cassini scientist Geraint Jones, of University College London, and is reported in the 7 March issue of the journal Science.

Rhea is roughly 1500 km in diameter and appears to be surrounded by at least one ring and a broad dusty disc extending up to 5900 km from the moon. The evidence for rings came from measurements made by Cassini’s Magnetospheric Imaging Instrument (MIMI). MIMI measured distinctive drops in the flow of electrons around Rhea. Interestingly, the drops occur symmetrically on either side of Rhea, suggesting the presence of rings circling the moon. The material making up the rings probably ranges in size from small pebbles up to boulders, and could be the remnant of a collision with an asteroid or comet in Rhea’s distant past.

“Seeing almost the same signatures on either side of Rhea was the clincher. After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon”
Geraint Jones, University College London